This invention relates to cryogenic refrigerators having linear drive motors such as the split Stirling refrigeration system shown schematically in FIG. 1. This system includes a reciprocating compressor 14 and a cold finger 16. A piston 17 of the compressor reciprocates in a cylinder 15 to provide a nearly sinusoidal pressure variation in a pressurized refrigeration gas such as helium. The pressure variation in a head space 19 is transmitted through a supply line 20 to the cold finger 16. The compressor piston 17 is driven by a linear drive motor including a permanent magnet 16 mounted on the piston and a drive coil 18 fixed to the cylinder 15.
Within the housing of the cold finger 16 a cylindrical displacer 26 is free to move in a reciprocating motion to change the volumes of a warm space 22 and a cold space 24 within the cold finger. The displacer 26 contains a regenerative heat exchanger 28 comprised of several hundred fine-mesh metal screen discs stacked within a cylindrical envelope to form a matrix. Other regenerators, such as those with stacked balls, are also known. Helium is free to flow through the regenerator between the warm space 22 and the cold space 24. A piston element 30 extends upwardly from the main body of the displacer 26 into a gas spring volume 32 at the warm end of the cold finger. The piston and displacer are driven by a linear drive motor including a permanent magnet 34 mounted to the piston and a drive coil 35. Detailed descriptions of the compressor and displacer drive motors can be found in my prior U.S. patent application Ser. No. 458,718, filed Jan. 17, 1983, for a Cryogenic Refrigerator with Linear Drive Motors.
Operation of the split Stirling refrigeration system will now be described. At the point in the cycle shown in FIG. 1, the displacer 26 is at the cold end of the cold finger 16 and the compressor is compressing the gas in the working volume. This compressing movement of the compressor piston 17 causes the pressure in the working volume to rise from a minimum pressure to a maximum pressure and thus warms the working volume of gas. Heat is given off to the environment from the compressor and the warm end of the cold finger. Thereafter, the displacer is moved rapidly upward. With this movement of the displacer, high-pressure working gas at about ambient temperature is forced through the regenerator 28 into the cold space 24. The regenerator absorbs heat from the flowing pressurized gas and thereby reduces the temperature of the gas.
The compressor piston 17 then begins to move up to expand the working volume. With expansion, the high pressure helium in the cold space 24 is cooled even further. It is this cooling in the cold space 24 which provides the refrigeration for maintaining a temperature gradient of over 200 degrees Kelvin over the length of the regenerator.
Finally, the displacer 26 is driven downward to the starting position of FIG. 1. The cooled gas in the cold space 24 is thus driven through the regenerator to extract heat from the regenerator. The heat added to the regenerator at an earlier time by high pressure working gas is less than the heat subtracted at this time by low pressure working gas. Therefore, there is net refrigeration.
The traditional approach to compressor drive motor design in split Stirling refrigerators has been to utilize a rotary electric drive in the compressor. Lubricated mechanical bearings and linkages are employed to convert rotary motion to oscillating motion. More recently, systems have been developed using a linear electric drive directly coupled to the compressor piston.
In order to provide an efficient refrigeration cycle, it is important that each of the compressor piston and cold finger displacer be driven full stroke in proper phase relationship to each other. To that end, a motor drive 36 supplies drive current to each of the coils 18 and 35 to establish the full stroke and the proper phase relationship. It is an object of this invention to provide a direct drive which maintains the proper strokes and phase relationship under varying operating conditions. It is particularly difficult to control the drive of the displacer because the displacer is also pneumatically driven by pressure differentials between the spring volume 32, the warm and cold volumes 22 and 24 and the volume in the regenerator 28. Several systems have been developed which rely on position feedback to provide more precise position control of the displacer and compressor. Examples can be found in U.S. Pat. Nos. 3,991,586 to Acord, 4,389,849 to Gasser et al., 4,397,155 to Davey, and 4,417,448 to Horn et al. Although such systems can increase the efficiency of a cryogenic refrigerator, they also add to the mechanical complexity of the system. This is a particular disadvantage with very small refrigerators.